1. Field of the Invention
The present invention relates to smoke detectors employing detection cells of the ionization type, and to the associated electrical circuitry for operation of the detection cell and for sensing the electrical changes which occur in the presence of airborne combustion products.
2. Description of the Prior Art
A smoke detection cell of the ionization type and circuits for d.c. operation of the detection cell are described in patent applications of Robert J. Salem, Ser. No. 630,204, filed Nov. 10, 1975, entitled "Smoke Simulating Test Apparatus for Smoke Detectors" and Ser. No. 630,202, filed Nov. 10, 1975, entitled "High Gain Sensing and Switching Means for Smoke Detectors", and assigned to the assignee of the present application.
A smoke detection cell of the ionization type suitable for use in the present application is described in said applications. It includes an alpha radiation source, such as a small quantity of Americium 241, in a measuring chamber having positive and negative electrodes. The measuring chamber ionizes the air between the electrodes, permitting the flow of a small electrical current when a d.c. voltage is applied across the electrodes. When airborne products of combustion (smoke) enter the measuring chamber, an increase in resistance to the flow of current is observed. The resulting change in the electrical conductivity of the measuring chamber is sensed and used to trigger an alarm when the change exceeds a given quantity. The latter quantity is selected to correspond to a level of smoke or aerosols within the measuring chamber representing a dangerous condition.
Electrical conductivity of the measuring chamber is sensed in said patent applications by measurement of the voltage across the measuring chamber, with the chamber being connected into a d.c. half-bridge. The other element of the half-bridge may be a resistance having a value comparable to that of the chamber or a second chamber from which airborne combustion products are excluded.